People in the past werent stupid.
Now why would I feel it necessary to say that? Because theres plenty of fiction, written and visual, where modern (or close to it) humans encounter more primitive humans. Sometimes its science fiction and fantasy methods like time travel; sometimes its the possibly mundane method of travelling to some less developed location, whether on this world or another one; but however it happens, its often apparent that the writer has confused the terms ignorant and stupid. Its also commonly seen when aliens come to Earth and proceed to go on at length about primitive humans.
Heres the difference: An ignorant person is someone who doesnt know something. On the other hand, a stupid person is, well, stupid.
Now of course the two are related; an ignorant person is likely to do something stupid because of something they didnt know, while a stupid person is likely to be ignorant of a great many things. The point is, were talking about two distinct sets which overlap, not two labels for the same set. The mistake too many writers make is assuming that the sets are the same thing and the words are synonyms.
Science-fictional example: In assorted Star Trek stories, the heroes encountered people from the past by either going there themselves or having the historical person transported forward in time. Those people are ignorant, at least initially, of a great many things that the main characters take for granted. Lily Sloane might not have known how phasers worked, and Zefram Cochrane might not have believed in aliens, but quite obviously neither was stupid. And witless certainly isnt a word one associates with Khan Noonien Singh, also a character initially (and very briefly) ignorant of the time he found himself in.
Now compare that to any story in which the befuddled natives stare in awe as the Mighty Whitey explorer holds up a lighter and proclaims himself a god and they believe him.
Theres an old story about a Spanish conquistador who arrives a river, and theres a native village on the other side. He has his local translator yell across the river, ordering the natives to come over with their canoes to transport his group over. The village chief questions why they should help this group of strangers (obviously carrying some kind of weapons, however unfamiliar). The conquistador proclaims himself the personification of the local sun deity and demands they obey him. The chief shouts back that if hes the sun god, clearly he should be able to dry up the river and walk across.
Yes, the story is more likely a joke rather than anything that actually happened, but its hardly beyond the realm of possibility that it could have happened. While people in the past may have had greater ignorance of many things, such as why exactly a woman became pregnant, or what caused earthquakes, and would associate those with the supernatural, that wasnt true of all things.
Oh, and given the belief in bunk like homeopathy, young earth creationism, Jenny Vaccines is eee-vil McCarthy, and a lot of the alleged medical advice Oprah has on her show, its not like weve got that much to crow about these days
Anyway, back to the past: When it came to human relations and politics, ancient peoples were hardly naive children who would need someone from the 21st century to explain how things worked. Sure,they might have paid lip service to their gods, but they did it in the same way that an 18th Century soldier might pray to their deity before a battle while still making sure to keep their powder dry.
A good example is the war between the Egyptians under Ramesses II (the Great) and the Hittites under Muwatalli II and later Hattusili III. Ramesses may have really believed he was a god-king, but he was conversant enough with his mundane reality that he knew when it was better to accept his human limitations and those of his people. The result was the first international peace treaty that we have a record of, having discovered the Hittite and Egyptian texts of it. (A replica hangs in the United Nations in the New York.) When you look at the treaty, you see things wed find familiar today; not only a cessation of hostilities, but also (a) a mutual defense alliance, an agreement to respond to the others aid in the event of foreign attack or internal insurrection, and (b) extradition agreements for criminals and traitors. Take away the language wed find ridiculously formalistic and self-congratulatory about how awesome the rulers involved were, and youd have the basics of a treaty that wouldnt be out of place today.
Another way of looking at is to consider the situation where you are the primitive being visited by the advanced aliens. Imagine some alien who shows you a device that, when he points it at something, the target erupts into flames! Wow! Magic stick, right? Of course not! A modern person, with the knowledge we have, would probably think theyve just seen a laser, or some other kind of beam weapon. They might well have gotten some of the details wrongthe device might not operate at all like a laserbut the point is, their first assumption is not going to be Its magicthat funny-looking creature must be a god!
As various people have pointed out, thats one of the problems a deity would face, were one to arrive today and try to convince people that he/she/it was actually a god via miracle. The effects would have to be pretty awe-inspiringnot because were smarter, but because our knowledge base is wider. As you go back in time, the knowledge base might narrow; but if the effect has some relation, however vague, to something the local people have familiarity with, they might not make that God did it jump that many writers assume.
Heres an example that I saw discussed a few years ago: Imagine a time traveler arrives around 10 CE (Common Era) with a bunch of modern firearms, and faces a Roman legion. At first glance, it looks like a pretty Wrath-of-God conflict, and in straight tactical terms it is; compared to what the Romans are using, the firearms have significantly longer aimed range, the gun user has higher mobility and rate of fire, and at medium and short ranges the bullets will blast through the shields and armour of the legionnaires with minimal trouble.
Would the modern-arms shooter be considered a god? No, he wouldnt. Take a look at it from the Roman point of view: At first they will no doubt be scared shitless by people being struck down out of nowhere, but a few minutes of observation shows that something made a hole and caused damage. Okay, making a hole and causing damage isnt exactly a concept the Romans are unfamiliar withits sort of the point (pardon the pun) of their way of making warso the cause of death is basically a known quantity. Further observation would show that the holes are on the side of the bodies facing the enemy, which indicates something coming from the enemy. Some kind of projectile; again, not a mysterious concept since, oh, pre-humans learned to throw rocks accurately. Given sufficient bodies, they will find a few of the projectiles, and will probably be able to identify the materials; lead, copper, steel. And at this point, they know theyre being hit by small, shaped metal projectiles, obviously traveling at great speednothing magical there. And if they happen to see a shooter, theyve got a pretty good idea what the weapon itself looks like.
Until they get their hands on a gun and its ammunition, theyre not going to figure out how the device propels the bullet with such speedhell, that bit could be magic, for all they knowbut the basic principle is familiar to people who routinely use ballistæ and the bow and arrow. And if the basic principle is understood, they definitely wont be too awestruck to do something about it: At the very least, theyll think about finding ways to stop the damn bullets. Keep in mind that the Romans were very, very good at defensive work, and that the basic principles of defense against bullets arent going to be significantly different than how theyd defend a fixed position from an enemy with contemporary weapons. Theres a reason the shovel is still part of the modern soldiers kit.
If our time traveler uses a nuke, well, now were in god territory, because the ancient Romans dont have the frame of reference, in human terms; they dont have any explosives, let alone anything that unimaginably destructive.
Now lets return to the deity showing up in the modern world:I can blow up planets!So can Darth Vader. Whats your point?"
Our frame of reference, even science-fictional, allows us a greater ability to place an effect into a context we can comprehend. So heres an obvious rule for deity wannabees: Avoid SF tropes. (Incidentally, this was the premise for Niven and Pournelles Inferno: An SF writer ends up in Hell, but he doesnt believe its really Hell Hell, just some Sufficiently Advanced Aliens jerking him around.)
For the writer, this gets back into worldbuilding. If you want the technologically (or magical equivalent) advanced entity to show up and impress the locals, you have to think carefully about what the locals would be familiar with, what your guy could do that the local chapter of the Skeptics Society would be able to provide a rational explanation for. Yes, their religious beliefs obviously have an influence, but remember that these people still live in a mundane world with things they dont find all that amazing. Also: More likely than not, at least some of them will be sufficiently skeptical of magic (even that of their own gods!) to question the new arrivals miracles. And even if they do believe, deeply and truly, in the paranormal and their gods, that can also work against you. To use an example from the Bible: If someone honestly believes that their god stopped the sun in the sky to prolong the day, at some point just blasting people with your BOOMSTICK! just isnt going to be that impressive, compared to what gods are supposed to do. Oh, sure, youve got some power but that makes you, what, maybe an angel of some kind, or more likely a minor demon?
You need to get inside the heads of the people, not just your god-impersonating character. And if you discover that the peoples logical reaction to someone holding up their rifle and proclaiming themself a god is to find out what happens when they fill the rifle-holders chest full of arrows you need to reconsider the impersonators strategy.